This invention relates to processing TV signals in the digital domain and more particularly to preventing/curing fine dot crawl along color transitions at vertical edges in displayed images.
The development of very large scale integrated (VLSI) circuits has made possible the processing of video signals in a TV receiver in the digital domain. As currently conceived, digital TV receivers will include an analog tuner and IF section since the RF signal frequency spectrum in these sections is too high for processing by current VLSI digital circuits. The IF video signal is detected and applied to an analog-to-digital (A/D) converter which generates a pulse code modulated (PCM) manifestation of the analog signal. In order to satisfy the Nyquist sampling criterion, and to phase lock the sampling clocks to the video signal, sampling is performed at some multiple of the color subcarrier frequency F.sub.sc. A sampling rate of 3F.sub.sc satisfies the criterion, however, to simplify some of the processing functions, it is more advantageous to sample at the rate of 4F.sub.sc.
The PCM composite signal is separated into luminance (Y) and chrominance (C) components which are separately processed and then combined in a matrix to produce red, green and blue (RGB) signals for application to the display kinescope. The chrominance signal is a phase and amplitude modulated signal comprising the linear sum of first and second carriers amplitude modulated with B-Y and R-Y signals, respectively, and wherein the first and second carriers have a 90 degree phase relationship. The R-Y and B-Y components of the chrominance signal are therefore in quadrature relationship. If the sampling rate of the A/D converter is at four times the chrominance subcarrier rate, the phase of the sampling points can be arranged so that successive PCM samples of the separated chrominance signal will contain only information of the respective color signals (B-Y) and (R-Y). That is, the successive samples of the separated chrominance signal will be interleaved in the form +(B-Y).sub.n, +(R-Y).sub.n, -(B-Y).sub.n, -(R-Y).sub.n, +(B-Y).sub.n+1, +(R-Y).sub.n+1, -(B-Y).sub.n+1, etc., where the subscripts identify the particular subcarrier cycle from which the sample is derived. Since the bandwidth of the color signals (B-Y), (R-Y) is considerably less than the carrier frequency, the (B-Y) and (R-Y) signals may be reconstructed with less than all the PCM chrominance samples. For example a (B-Y) demodulated signal may be generated from the separated chrominance signal by demultiplexing the +(B-Y) samples only, and similarly a demodulated (R-Y) signal can be produced by demultiplexing the +(R-Y), samples only.
It will be recalled that the chrominance carrier is 180 degrees out of phase from scan line to scan line. Thus, for an NTSC system where the subcarrier F.sub.sc is 3.58 MHz and the sample rate is 4F.sub.sc, the +(B-Y) samples, and similarly the +(R-Y) samples, are displaced by 140 ns line-to-line. The present inventor has determined that this 140 ns offset gives rise to a fine dot crawl along vertical edges, that is, vertical edges appear to be serrated with the serration creeping upward on the display.
Accordingly, it is an object of the present invention to provide means to demodulate the chrominance signal by simple demultiplexing while not introducing the fine dot crawl.